a) Field of the Invention
The present invention relates to a viewfinder optical system for single-lens reflex cameras, and more specifically to a viewfinder optical system which uses an eyepiece lens system permitting adjustment of diopter.
b) Description of the Prior Art
In the recent years where single-lens reflex cameras have a large number of functions, viewfinders are used for displaying increasing numbers of information items outside visual field frames. Under this circumstance, it is considered that one of important functions of viewfinders is to permit clearly confirming not only visual fields but also the information items. However, most of the conventional viewfinders for single-lens reflex cameras cannot provide sufficiently clear appearances of marginal portions and surroundings of visual fields including the information items, and photographers, in particular those who use spectacles, strongly desire improvements of the conventional viewfinders. As a viewfinder which can solve this problem, there is known the so-called high eye point type viewfinder having a long distance (an eye relief) which is as measured from a vertex of a lens surface of an eyepiece lens system disposed on the side of an eye of a photographer to an eye point of the viewfinder optical system so that an overall range of a visual field can be observed with no eclipse even when the eye is located more or less apart from the eyepiece lens system or when the photographer uses spectacles.
On the other hand, viewfinders for single-lens reflex cameras are generally configured so as to set locations of images to be observed through eyepiece lens systems, or diopter, in the vicinity of -1, thereby making it difficult for long-sighted and strongly short-sighted photographers to bring the cameras into good focus and read the information items clearly. It is therefore desirable that viewfinders for single-lens reflex cameras are of the type which permits adjustment of diopter.
As a conventional example of viewfinder optical systems which permit adjustment of diopter, there is known, for example, a viewfinder optical system disclosed by Japanese Patent Kokai Publication No. Sho 54-126,530. This viewfinder optical system adopts an eyepiece lens system which consists, in order from the side of an eye of a photographer, of a positive lens component and a negative lens component, and is configured so as to permit adjustment of diopter by moving either of these lens components along an optical axis. Further, there are known, as improved versions of the viewfinder optical system described above, there are known viewfinder optical systems each of which consists of a negative lens component, a positive lens component and a negative lens component as exemplified by viewfinder optical systems disclosed by Japanese Patents Kokai Publication No. Sho 56-91,210, No. Sho 59-148,021, No. Sho 61-156,017 and No. Sho 64-81,925. Out of these viewfinder optical systems, those disclosed by Japanese Patents Kokai Publication No. Sho 59-148,021 and No. Sho 64-81,925 are known as high eye point type.
However, the conventional high eye point type viewfinder optical systems which permit adjustment of diopter have a common defect that they cannot provide clear appearances of marginal portions of visual fields.